After considering the issue for a while, I think I would like to see
finalized libsvn_repos function signatures use an abstract type for the
authz configuration. It can just contain an svn_config_t * in the
current implementation.

My reasoning is that the current representation does not seem to be as
efficient as I thought it was. We appear to enumerate and reparse the
config file for each access check, and I suspect we can do substantially
better with a different representation. I think we should leave the
door open to that.

(Also, it looks like we're already supporting recursive queries of the
authz configuration. So, everything I said before about having to give
up on wildcard support is moot; apparently, we've already given up on
that.)